1. Field of the Invention
The invention relates to an electron-optical system having a magnetic focusing and an electromagnetic deflection system of unit design and a screening means between these two arrangements in the form of a coil of magnetic wire or other conventional structure, and is particularly concerned with an electron-optical system of this type for high resolution television camera tubes such as vidicon tubes or the like which have a target for scanning by an electron beam and a fine-mesh field net arranged in front of the target.
The invention is of special significance to all television camera tubes, in particular to vidicons as used in single tube color television cameras which operate both with magnetic focusing and electromagnetic deflection.
2. Description of the Prior Art
Vidicon type television camera tubes are operated predominantly with magnetic focusing and electromagnetic deflection because the lens aberrations and the beam divergence at the photosensitive layer are small as a function of the deflection angle, in contrast to the case with electrostatic focusing and deflection. Of course, when using magnetic deflection and focusing units, in association with the relevant electrostatic fields in the particular electron tube, beam landing errors occur which can give rise to considerable disturbances and must, therefore, be made as small as possible.
In this context, the term "beam landing errors" is intended to describe the following effect. The electron beam, because of the deflection it undergoes, strikes the photosensitive layer obliquely, unless additional corrective measures are taken, i.e. because of this phenomenon occurring with magnetic deflection, its axial velocity component is less than if it were perpendicularly incident, e.g. at the picture center, so that the degree of deviation from the perpendicular beam landing increases outwardly of the picture center. On striking the target, the electrons negatively charge the target surface until, the axial electron velocity is no longer sufficient. The potential build up of the electron beam is accordingly less negative at the picture edge, due to oblique incidence of the beam, than it is at the picture center. The potential difference between an arbitrary point on the target, and the picture center, is referred to as the beam landing error and is therefore quoted in volts. With normal coil units and with the camera tube operating under normal conditions, this error amounts to up to 2 volts. Because of the beam landing error, the electron field within the photosensitive layer, i.e. between the light entry side and the scanning side, or between front and rear sides of the target, differs at two points on the target area which are differently located visa-vis the picture center, because at the light entry side of the target, facing away from the electron beam, the surface potential is locally constant. Local differences in the field strength within the layer are not only responsible for a position-dependent variation in the dark current, but also for a corresponding variation in sensitivity. This phenomenon is particularly marked in targets having Sb.sub.2 S.sub.3 or Se layers. Consequently, pictures taken under these conditions exhibit unwanted shadowing.
In the "Journal of SMPTE", Vol. 68, April 1959, at Pages 226-229, I. Castleberry and B. H. Vine describe a magnetic deflection and focusing unit for camera tubes. This known coil unit has a conventional elongate focusing coil whose overall uniform field along the axis is modified simply towards the target end, by an additional (focus boost) coil, so that the beam landing error is canceled out due to the resultant steep field gradient. This known coil unit, however, has three critical drawbacks which have heretofore prevented its application. These are:
1. Because the axial extent of the deflection field is very small, the coil has a very low deflection sensitivity so that expensive deflection amplifiers are required whose linearity, in many cases, is insufficient for normal picture quality requirements.
2. The focusing coil produces a virtually uniform axial field so that no reducing reproduction of the aperture diaphragm on the target, by the electron beam, is possible. However, in a high-resolution camera tube, it is precisely a reducing reproduction characteristic which is required because the aperture diaphragm, in view of the requisite beam current, cannot be reduced in size below a diameter of 10 .mu.m.
3. Because a steep gradient in the axial focusing field component must be achieved in the vicinity of the target, an additional coil, a focus boost coils, produces an axially tightly restricted additional field. This additional or focus boost coil is, on the one hand, technically elaborate and furthermore, in the selected arrangement, has the drawback that picture distortion can occur.